The present invention relates to an ultrasonic motor and a method of adjusting the same and, more particularly, to an ultrasonic motor for generating ultrasonic vibrations in a transducer as a stator so as to rotate a rotor urged against the stator by using a frictional force and a method of adjusting the same.
In a conventional ultrasonic motor of this type, a rotor is urged against an end face of a stator having a longitudinal-torsional composite transducer, and the rotor is rotated by a frictional force (Japanese Laid-Open No. 61-12177).
FIG. 1 is a sectional view showing a conventional ultrasonic motor. As shown in FIG. 1, in the conventional ultrasonic motor, a stator is integrally formed by sandwiching a torsional vibration exciting piezoelectric element 101 (arrows indicate polarization directions) and a longitudinal vibration exciting piezoelectric element 102 between columnar or cylindrical ultrasonic vibrators 103 and 104, and is clamped by a bolt 106. In addition, the ultrasonic motor has a rotor 105 urged against an end face of the stator. An elliptical motion is generated on the end face of the stator by independently applying voltages to the piezoelectric elements 101 and 102. The rotor 105 is rotated by utilizing this elliptical motion.
Referring to FIG. 1, reference numerals 107, 108, 109, and 110 denote terminal plates.
Since the ultrasonic speed of a torsional vibration wave is about 60% of that of a longitudinal vibration wave, it is difficult to match the resonance frequency of torsional vibrations with that of longitudinal vibrations in the stator having the above-described arrangement. Therefore, in the above stator, if the stator is resonated and driven to generate torsional vibrations, longitudinal vibrations are generated by nonresonant driving. On the contrary, if longitudinal vibrations are generated by resonant driving, torsional vibrations are generated by nonresonant driving.
As is known, in comparison with resonant driving, an amplitude obtained by nonresonant driving becomes very small if the power remains the same. As a result, the vibration amplitude in either the longitudinal or torsional direction of the elliptical motion generated on the end face of the stator becomes small, and hence a high-efficiency ultrasonic motor is difficult to realize.
In addition, in the stator having the above-described arrangement, two types of vibrations can be generated by resonant driving by applying AC voltages having different frequencies to the torsional vibration exciting piezoelectric element and the longitudinal vibration exciting piezoelectric element. In this case, however, since resonance frequencies are different from each other, an elliptical motion cannot be regularly generated on the end face of the stator, and the rotor cannot be stably rotated.